Cryptococcus neoformans is one of the most serious fungal pathogens affecting primarily immunocompromised individuals such as those with AIDS. It is a unicellular budding yeast which is surrounded by a thick polysaccharide capsule. The fungus is neurotropic and causes meningoencephalitis which is fatal unless treated. The organism exists in two mating types: 'a' type and alpha type. Although the two types survive equally well in thelaboratory, the alpha type is always predominant among clinical isolates as well as among environmental isolates. In an isogenic set of a mating type and alpha mating type, it has been shown that the alpha mating type is more virulent in the mouse model. Considerable progress in the identification of genes associated with pathogenicity and virulence of C. neoformans has been made in this laboratory for the past 7 years. During 1993-1999, four capsule genes have been identified and characterized as essential for virulence of the organism. This year we have initiated molecular dissections of the mating type specific genes that regulate the expression of virulence associated genes such as the CAP genes. In order to accomplish this purpose, we first identified and mapped the boundary of the alpha mating type alpha locus. We have detected three MAP kinase cascade genes such as the STE11alpha, STE20alpha, and STE12alpha within the locus and cloned two additional mating type alpha specific genes, CPR1alpha and Myosine. The STE12alpha gene was found to regulate the expression of virulence associated genes such as capsule genes. Deletion of the STE12alpha gene resulted in a significant decrease in capsule formation and virulence of C. neoformans.